In the field of cash registers, the printing performed by a print head is commonly registered. When a dot matrix impact printer is used for printing receipts, printing is performed simultaneously on two tapes, with the impact on the first tape marking the second which is then wound up and stored to constitute a record or journal of all the transactions performed by the machine.
With a thermal printer, the problem is different since there is no impact whereby marks can be made on a secondary medium. Two media are therefore used. In a first technique, the first medium may be fine thermal paper enabling heat to be transferred to a second paper that is heat-sensitive. In another version, the first paper is heat-sensitive on its front face and forms a transfer film on its back face to transfer ink onto normal paper that constitutes the second print medium.
Using two media in, and in association with, a thermal print head always constitutes a complication. It is necessary to ensure that each medium is guided separately since the media have different destinations.
Furthermore, thermal printing by a technique in which ink is transferred from an ink-carrying film onto a medium (e.g. ordinary paper) can be adapted to printing a journal of operations only by duplicating the print head and causing the heads to print identically and simultaneously. In addition to being expensive, that technique is not guaranteed to provide an exact record of the operations performed by the printer. One of the printers may not be operating in a manner that is identical to the other, in which case their printouts may be different.